Ubiquitin is a small protein that has important regulatory roles in a wide variety of cellular pathways. The best known of these is ubiquitin's role in protein degradation, where covalent attachment of ubiquitin to a target protein enables that target protein to be recognized and destroyed by the 26S proteasome (see Wilkinson, Semin. Cell Devel. Biol. 11(3): 141-148 (2000)). Protein kinase regulation of various signaling pathways has also been correlated with ubiquitination (see Sun and Chen, Curr. Opin. Cell Biol. 16: 119-126 (2004)). For example, phosphorylation of IκB by IκB kinase permits ubiquitination of IκB and subsequent degradation by the 26S proteasome; because IκB is an inhibitor of NFκB, the degradation of IκB activates NFκB (Ghosh and Karin, Cell 109 (Suppl.): S81-S96 (2002); Palombella et al., Cell 78: 773-785 (1994)). Ubiquitination also mediates DNA repair (see Sun and Chen, Curr. Opin. Cell Biol. 16:119-126 (2004)). After DNA is damaged, monoubiquitination of proliferating cell nuclear antigen (PCNA) activates damage-tolerant polymerases which are able to synthesize DNA despite any DNA lesions (Stelter and Ulrich, Nature 425: 188-191 (2003). Other physiological processes in which ubiquitination is known to be involved include cell division, cell growth, cell movement, and apoptosis/cell death (Johnson, Nat. Cell Biol. 4:E295-E298 (2002); Pickart, Mol. Cell. 8: 499-504 (2001)).
The covalent attachment of ubiquitin, a 76 amino acid protein, to a target protein is a three-step enzymatic process (Pickart, Annu Rev. Biochem. 70: 503-533 (2001)). First, ubiquitin-activating enzyme E1 forms an ubiquitin-E1 thioester in an ATP-dependent reaction. The ubiquitin is transferred from the ubiquitin-E1 thioester to a member of the ubiquitin-conjugating enzyme (E2) family in the second step. In the third step, with the assistance of a ubiquitin-protein ligase (E3), an isopeptide bond is formed between the carboxyl terminus of ubiquitin and the ε-amino group of a lysine residue on the target protein. Enzymes termed deubiquitinases remove ubiquitin moieties from target proteins (Guterman and Glickman, Curr. Prot. Pep. Sci. 5: 201-210 (2004)). Highlighting ubiquitin's role as an important regulatory molecule, the human genome contains many different proteins involved in ubiquitination or deubiquitination: at least 40 different E2s, 500 different E3s, and 80 different deubiquitinases have been identified thus far (Wong et al., Drug. Discov. Today 8: 746-754 (2003)).
Ubiquitin contains seven lysine residues (Lys6, Lys11, Lys27, Lys33, Lys29, Lys48, and Lys63), and thus ubiquitin itself may serve as a target protein for ubiquitination (Peng et al., Nat. Biotechnol. 21: 921-926 (2003); Pickart and Fushman, Curr. Opin. Chem. Biol. 8:610-616 (2004)). The molecule produced upon ubiquitination of a ubiquitin protein is termed a polyubiquitin molecule, and may comprise two or more ubiquitin moieties. Ubiquitination of ubiquitin may theoretically occur at any of the seven lysine residues (Peng et al., Nat. Biotechnol. 21: 921-926 (2003)), so that different species of polyubiquitins exist having isopeptide bonds to different lysine residues within ubiquitin. It is possible that a single polyubiquitin molecule with greater than two ubiquitin moieties may have more than one type of lysine linkage. Studies have shown that the E2 enzyme influences the type of lysine linkage created between one ubiquitin molecule and another (Tenno et al., Genes to Cells 9: 865-875 (2004); Deng et al. (2000); Hofmann and Pickart (2001)). Polyubiquitin and ubiquitin exist both as free molecules and in covalent attachment with a target protein.
Like ubiquitin, polyubiquitin involvement has been found in many cellular processes, including intracellular trafficking, endocytosis, gene expression/silencing, proteolysis, kinase activation, translation, and DNA repair (Hoege et al., Nature 419:135-141 (2002); Spence et al., Mol. Cell. Biol. 15:1265-1273 (1995); Hofmann and Pickart, Cell 96: 645-653 (1999). Polyubiquitin and polyubiquitination can have strikingly different physiological roles than monoubiquitin and monoubiquitination in the same pathways, however. For example, whereas monoubiquitination of PCNA after DNA damage results in the activation of error-prone DNA polymerases, polyubiquitination of PCNA at the identical residue where monoubiquitination is observed results in activation of error-free DNA repair (Stelter and Ulrich, Nature 425: 188-191 (2003); Hoege et al., Nature 419:135-141 (2002); Spence et al., Mol. Cell. Biol. 15:1265-1273 (1995); and Hofmann and Pickart, Cell 96: 645-653 (1999)).
Even polyubiquitins having different lysine linkages appear to play different physiological roles. The two best-studied are the Lys48-linked and Lys63-linked polyubiquitins, and structural studies of the two suggest that different lysine-linked polyubiquitins may adopt markedly different conformations, thus permitting different interactions with selected binding partners (Tenno et al., Genes to Cells 9: 865-875 (2004)). Covalent modification by Lys48-linked polyubiquitin typically marks the target protein for proteolytic degradation, though there is some evidence that Lys48-linked polyubiquitin may also regulate certain proteins by non-proteolytic means (Chau et al., Science 243: 1576-1583 (1989); Finley et al., Mol. Cell. Biol. 14: 5501-5509 (1994); Flick et al., Nat. Cell. Biol. 6:634-641 (2004)). Lys63-linked polyubiquitins, in contrast, have been linked to a variety of nonproteolytic intracellular pathways, including DNA repair (yeast cells expressing K63R-ubiquitin are defective in DNA repair), kinase activation, intracellular trafficking, and translation (Pickart and Fushman, Curr. Opin. Chem. Biol. 8: 610-616 (2004); Hicke and Dunn, Annu Rev. Cell Dev. Biol. 19: 141-172 (2003); Spece et al., Mol. Cell. Biol. 15: 1265-1273 (1995); Ulrich, Eukaryot. Cell 1:1-10 (2002); Spence et al., Cell 102: 67-76 (2000); Seibenhener et al., Mol. Cell. Biol. 24(18): 8055-8068 (2004)). In one specific example, synphilin-1 is normally ubiquitinated with K63-linked polyubiquitin by parkin in a proteasomal-independent manner, but synphilin-1 can alternately be targeted for destruction by ubiquitination with K48-linked polyubiquitin (Lim et al., J. Neurosci. 25(8): 2002-9 (2005)). An analysis of subjects with Parkinson's disease shows that K63-polyubiquitination of synphilin-1 may be involved in the formation of Lewy body inclusions associated with that disease (Lim et al., J. Neurosci. 25(8): 2002-9 (2005)).
Other lysine-linked polyubiquitins have not been studied extensively, largely because of the difficulty in distinguishing between them. Studies have thus far relied on cells expressing mutagenized ubiquitins in which one or more lysines have been removed, on enzymatically synthesized polyubiquitins of particular linkages, or on techniques such as mass spectrometry to distinguish between one type of polyubiquitin and another. Each of those methodologies is ill-suited or cumbersome for analysis of the normal physiological behavior of particular lysine-linked polyubiquitins. While antibodies exist that are specific for polyubiquitin as opposed to monoubiquitin (Fujimoro et al., FEBS Lett. 349: 173-180 (1994)), there are as yet no antibodies that can distinguish between polyubiquitins of different lysine linkages.
Unsurprisingly, given their important roles in a variety of cellular processes, ubiquitin and polyubiquitins have also been implicated in many diseases (see Argiles, Ubiquitin and Disease, R. G. Landes (1998)). Ubiquitin dysregulation is observed in muscle wasting (Mitch and Goldberg, New Engl. J. Med. 335: 1897-905 (1996); Bodine et al., Science 294: 1704-1708 (2001)). Several genetic diseases have been linked to aberrant ubiquitin activity, including cystic fibrosis (Ward et al., Cell 83: 121-127 (1995)), Angelman's syndrome (Kishino et al., Nature Genet. 15: 70-73 (1997)), and Liddle syndrome (Staub et al., EMBO J. 16: 6325-6336 (1997)). Ubiquitin also plays a role in immune and inflammatory responses; for example, extracellular ubiquitin has been found to act as a sort of cytokine, inhibiting the TNFα response to endotoxin in peripheral blood mononuclear cells and regulating endotoxin hyporesponsiveness (Majetschak et al., Blood 101: 1882-1890 (2003); Ciechanover, EMBO J. 17: 7151-7160 (1998)). Also, both ubiquitin and polyubiquitin have been found in human serum, with higher levels of both molecules observed in the serum of patients having parasitic and allergic disease (Takada et al., Clinical Chem. 43: 1188-1195 (1997)).
Dysregulation of several ubiquitin-mediated pathways are also involved in cancer (Spataro et al., Br. J. Cancer 77: 448-55 (1998); Beckmann et al., Hum. Mutat. 25: 507-12 (2005)). For example, mutations in the heterodimeric ubiquitin ligase BRCA1-BARD1 are correlated with breast cancer (Hashizume et al., J. Biol. Chem. 276: 14537-40 (2001)), mutations that disrupt the ability of Myc to be degraded by the ubiquitin pathway activate the oncogenic potential of c-Myc (Salghetti et al., EMBO J. 18: 717-726 (1999)), and transformed v-Jun is unable to be ubiquitinated and degraded as its non-oncogenic correlate, c-Jun, is, giving rise to uncontrolled growth (Ciechanover, EMBO J. 17: 7151-7160 (1998); Trier et al., Cell 78: 787-798 (1994)).
Ubiquitin and polyubiquitin have particularly been studied in the context of neurological diseases (Chung et al., TINS 24(11 Suppl.) S7-S14 (2001)). The inclusions, bodies, and neurofibrillary tangles that accumulate in Huntington's disease, Spinocerebellar ataxia, prion encephalopathies, Pick's disease, Lewy body disease, Parkinson's disease, and Alzheimer's disease stain immunopositively for mono and/or polyubiquitin (Alves-Rodrigues et al., Trends Neurosci. 21: 516-520 (1998); Cammarata et al., Neurosci Lett. 156: 96-98 (1993); Kalchman et al., J. Biol. Chem. 271: 19385-94 (1996); Holmberg et al., Human Mol. Genet. 7: 913-918 (1998); Yedidia et al., EMBO J. 20: 5383-91 (2001); Mori et al., Science 235: 1641-44 (1987); Leigh et al., Acta Neuropathol. (Berl.) 79: 61-72 (1989); and Kuzuhara et al., Acta Neuropathologica 75: 345-353 (1988)). Several forms of Parkinson's disease have been linked to mutations in the ubiquitin carboxy-terminal hydrolase L1 (UCH-L1) gene, a deubiquitinase (Leroy et al., Nature 395: 451-452 (1998)), while other forms of Parkinson's have been linked to inactivating mutations in Parkin, an E2-dependent ubiquitin-protein ligase known to interact with the ubiquitin-conjugating enzyme UbCH7 and to ubiquitinate synphilin-1 (Shimura et al., Nature Genet. 25: 302-305 (2000), Zhang et al., Proc. Natl. Acad. Sci. 97: 13354-13359 (2000); Lim et al., J. Neurosci. 25(8): 2002-9 (2005)). Both types of mutations result in aberrant proteolytic processing and the inappropriate aggregation of proteins (see McNaught et al., Nature Rev. Neurosci. 2: 589-594 (2001)). UCH-L1 mutations have also been found to segregate with Huntington's disease (Naze et al., Neurosci. Lett. 328:1:1-4 (2002)). A mutant form of ubiquitin has been identified in the brains of Alzheimer's patients that is very efficiently incorporated into polyubiquitin chains, but is refractory to deubiquitination once formed, potentially leading to dominant inhibition of the normal cellular proteolytic processing system (Lam et al., Proc. Natl. Acad. Sci. 97: 9902-9906 (2000)).
It is clear that it would be beneficial not only to have compositions and methods that can distinguish between polyubiquitins of different lysine linkages, but also to have compositions and methods that are effective in targeting and modulating ubiquitin and polyubiquitin-mediated pathways. The invention provided herein relates to such compositions and methods.
All references cited herein, including patent applications and publications, are incorporated by reference in their entirety.